Refers to the person or entity that dispatches or sends goods, mail, etc. Common in logistics and postal contexts.
The sender gave me the tracking number for the package.
返品の際は、発送者の住所を明記してください。
When returning the item, please clearly write the sender's address.
差出人 is also 'sender', but more common in postal mail contexts. 発送者 is broader, covering shipping of goods.
Compound of 発送 (dispatch, shipping) and 者 (person).